1. Field of the Invention
This invention relates generally to soft, flexible, dry-formed fibrous sheets having an aesthetically pleasing appearance for use in many applications for which conventional textile fabrics have been employed in the past, and to a unique method for manufacturing such sheets. More specifically, this invention relates to dry-formed, adhesively bonded nonwoven fibrous sheets which are embossed, bonded and creped in a manner to achieve sufficient strength properties to permit their use for applications in which conventional textile fabrics have been employed in the past, and in addition to possess a high degree of bulk and aesthetic appeal which enhances their suitability for replacing such conventional textile fabrics.
2. Description of the Prior Art
Dry-formed, nonwoven sheets have become popular, especially for single and limited use applications, to replace higher cost textile fabrics formed by conventional textile operations, such as weaving and knitting. Obviously, it is highly desirable to form nonwoven sheets in a manner which will impart properties to them that are similar to those possessed by conventional textile fabrics which said sheets are intended to replace. Specifically, such nonwoven sheets should be formed in a manner to impart sufficient strength, bulk, flexibility, extensibility and softness thereto so that the nonwoven sheet will approximate the "hand" and appearance of such textile fabrics.
Nonwoven sheets have already been accepted as replacements for conventional textile fabrics for many end uses. For example, absorbent nonwoven fibrous sheets are commonly employed today as replacements for conventional textile fabrics for household and industrial wipers, as well as for cover sheets in sanitary napkins and disposable diapers.
Dry-formed nonwoven fibrous sheets have been manufactured by a wide variety of processes. In forming such nonwoven sheets considerable attention has been directed to imparting to the sheets both physical and aesthetic properties which are similar to the physical and aesthetic properties of the textile fabrics which said sheets are intended to replace. Properties such as porosity and/or absorbency are highly desirable in nonwoven fibrous sheets which are employed in products such as household wipers, industrial wipers and cover sheets for sanitary napkins and disposable diapers. However, it is well known that for the general replacement of textile fabrics, the most critical properties which nonwoven sheets must possess are softness and an aesthetic appearance approximating such textile fabrics.
The prior art has suggested many methods for enhancing the softness of dry-formed nonwoven sheets. These methods have included, in part, the judicious selection of specific elastomeric binders to bond the fibers in the web together; the application of binders in specific spaced-apart patterns and various post-treatment operations, such as embossing and creping.
Creping is one of the most commonly employed techniques for enhancing the softness of a fibrous web; the prior art relating to creping being quite extensive. Known creping techniques include the conventional blade creping of a fibrous sheet from a creping surface, as well as compacting techniques employing flexible belts, rolls provided with flexible coverings and combinations of such rolls and belts. These methods have all been designed to deform the fibrous sheet, by compaction, in a manner to work the fibers in the sheet for enhancing softness.
Several techniques are also known for enhancing the appearance, or texture, of nonwoven sheets so that the sheets will approximate the appearance of conventional textile fabrics which they are intended to replace. One technique which has been commonly employed for this purpose is to include apertures or holes in the web, and this approach has been disclosed in many patents. In addition, the prior art discloses a number of techniques for creping, or consolidating a nonwoven sheet to achieve a texture that is more textile-like than the familiar cross-ridged configuration of conventional creped sanitary paper products, such as facial tissue, toilet tissue and the like.
British Pat. No. 1,294,794, assigned to Scott Paper Company, discloses controlling the crepe pattern in a nonwoven web containing predominantly short cellulosic fibers of a papermaking length less than about 6 millimeters by adhering the web to a creping surface through a spaced-apart pattern of a binder which is applied to the web prior to or at the same time as adhering said web to the creping surface. The web is adhered to the creping surface either solely, or most tenaciously in the web regions occupied by the binder, and the specific binder pattern functions to control, or regulate the overall appearance, or crepe pattern, in the completely formed nonwoven sheet.
U.S. Pat. No. 3,665,921, relates to the mechanical working of a nonwoven fibrous web, and is one of a related family of patents owned by Kimberly-Clark. This patent discloses a consolidating operation in which the web is bonded by a thermoplastic adhesive in a spaced-apart pattern, and then is consolidated by being removed from a creping drum by a consolidating blade to coalesce the spaced-apart adhesive pattern to produce a looped fiber structure.
U.S. Pat. No. 3,301,746, issued to Sanford et al., teaches the adherence of a thermally predried paper web to a creping surface by pressing the web against the surface with a high-knuckle carrier fabric; the knuckle pattern influencing the crepe pattern resulting from the removal of the web from the creping surface by a creping doctor blade.
U.S. Pat. No. 3,059,313, issued to Harmon, discloses the post treatment of a fibrous web which has a binder applied to it in a discrete, spaced-apart pattern. This web is post-treated by crimping the fabric between opposed elastic roll surfaces to deform the web into an undulating configuration which is controlled by the spaced-apart binder segments. Specifically, the binder segments are disposed in side walls of the undulations intermediate the peaks of said undulations.
Other methods have been proposed for controlling the behavior of a fibrous web as it is creped from a creping surface. For example, it has been suggested to crepe a fibrous web from a creping surface with a serrated creping doctor blade to achieve a ribbed appearance in the web.
Many of the above-discussed prior art processes are limited in their ability to either affect a uniform softening of a nonwoven sheet, or to achieve a wide variety of different appearances, or textures in such a sheet. For example, there are definite limitations as to the manner in which a binder can be applied to a nonwoven web in order to impart the requisite strength to the completed sheet. The most severe limitation is dictated by the particular fiber composition of the web, since the adhesive should be applied in a manner to stabilize the fibers to a degree sufficient to permit the use of the sheet as a replacement for a conventional textile fabric. Accordingly, when a spaced-apart binder pattern is employed to control the configuration of a web resulting from a creping or compacting operation, the particular appearances which can be achieved are limited by the particular fiber composition of the web, since the fiber composition dictates the particular bond spacing which can be tolerated.
When a serrated doctor blade is employed to control the configuration of a nonwoven web in a creping operation, regions of the web which are aligned with the serrations in the doctor blade will not experience the same high degree of softening as the web regions which are directly contacted by other regions of the doctor blade. Accordingly, uniformity in softness is not always achieved by this method of creping and the appearance is limited to stripes.
When the appearance of a web is controlled by applying a differential pressure to different regions of a web by a highknuckle fabric to adhere said web to a creping surface for subsequent creping, the pattern, or appearance of the creped web is influenced by the particular knuckle pattern in the fabric. Generally, the configuration of the knuckle pattern is limited by the particular mesh of the fabric which is required to support said web so that it will not become damaged by being trapped within the interstices between elements of said fabric. Accordingly, the control over the final appearance of the web is limited, at least in part, by the particular spacing which can be tolerated between the elements of the fabric.